Music Theory Resources:
A few resources those who study music or music theory may be interested in:
A program to explore and learn music theory, using the concepts I write about here.
To learn more about tuning and how it may not follow the Circle of Fifths as one would expect, search "meantone" to find many excellent articles. I don't have any one particular article I like more than others. Just be prepared for some fairly technical articles.
Entertaining / Thought-Provoking:
Solo Guitar Playing - Book 1, 4th Edition by Frederick Noad (Not exactly music theory, but it is the best book on learning guitar notation that I'm aware of)